1. Field of the Invention
The invention relates to acid fracturing fluids used to fracture subterranean formations. Specifically, the invention relates to methods for breaking an acid fracturing fluid through the use of a liquid breaker incorporated in the acid fracturing fluid.
2. Description of the Related Art
Subterranean formations of oil and gas wells are often treated by fracturing the formations to increase the production of oil or gas. Acid fracturing is a type of well stimulation treatment performed in carbonate formations in which acid is forced into the formation at a pressure above the formation-fracturing pressure to cause the formation to crack open. The acid reacts with the fracture faces in a non-uniform pattern, which is referred to as etching, to form conductive channels that remain open without a propping agent after the fracture closes.
Acid fracturing generally requires the fluid to reach maximum viscosity as in enters the fracture, which affects the length and width of the fracture. Many types of acid systems can be used for acid fracturing, but crosslinked acid is one of the most commonly used systems. In the crosslinked acid system, a polymer is hydrated and gelled in an acid, then crosslinked through crosslinking using metal crosslinking agents to increase its viscosity. The viscosity of the acid fracturing fluid correlates to the created fracture geometry and fracture width. Normally, the more viscous fluids will produce longer and wider fractures. Crosslinked acid systems have advantages over other prior art acid fracturing treatments, such as deeper penetration into the formation, and superior acid leakoff control.
Once the acid fracturing treatment is complete, the acid fracturing fluid is recovered by reducing the viscosity of the fluid by means of “gel breakers.” Gel breakers break down the gelled fluid so that it can be easily pumped and removed from the well. The recovery of the acid fracturing fluid is accomplished by reducing the viscosity of the fluid to a low enough viscosity so that it flows naturally from the formation.
Various methods have been proposed to control the break mechanism of the prior art breaker systems. Most prior art methods of breaking acid fracturing fluids include the use of solids, such as prills, pellets, beads, capsules, granules, encapsulated materials, and the like. Often times these types of breakers are difficult to handle and do not disperse well. Due to operational limitations, breakers in solid form may not be applicable in field applications. Breakers in solid form may also be unable to break the polymer gels penetrating deep into the formation.
Besides issues related to handling of breakers, the timing of the breaking of the fracturing fluid is important. Solid breakers often do not produce consistent breaking times for the acid fracturing fluids. Breaking too soon or too late causes problems with the fracture. Gels that break prematurely can cause a premature reduction in the fluid viscosity resulting in a less than desirable fracture width in the fracture being created. On the other hand, crosslinked fluids that break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids.
A need exists for a breaker that is effective and relatively easy to handle. It would be advantageous if the breaker is efficient at reducing viscosity of acid fracturing fluids in a controlled manner. It would be also desirable to have a “live” breaker that can break the polymer gels penetrating deep into the formation.